1. Field of the Invention
The present invention relates to an image processing system arranged to perform a gradation conversion on an image signal and a recording medium recording an image processing program for performing the gradation conversion on the image signal.
2. Description of the Related Art
As a gradation processing to be performed on an image signal, a space-invariant method of using a single gradation conversion curve for the image signal and a space-variant method of using a plurality of gradation conversion curves different for each local region are proposed.
For example, Japanese Patent No. 3465226 discloses a technology for dividing the image signal into a plurality of regions on the basis of texture information, performing a gradation conversion processing by calculating gradation conversion curve for each region on the basis of a histogram, and performing a weighting interpolation on the basis of a distance between the respective regions. With the configuration, it is possible to perform the space-variant gradation processing and maintain the continuity between the regions, and it is possible to obtain the high quality image signals in which light-dark crush is prevented also for an image having a wide dynamic range.
Also, Japanese Unexamined Patent Application Publication No. 8-56316 discloses a technology for separating the image signal into a high frequency component and a low frequency component, performing a contrast emphasis processing on the low frequency component, and synthesizing the low frequency component after the contrast emphasis processing with the high frequency component. By employing such a technology, an emphasis on the noise of the high frequency component is prevented, and it is possible to obtain the high quality image signals.
Furthermore, Japanese Unexamined Patent Application Publication No. 2004-128985 discloses a technology for estimating a noise amount for each block unit on the basis of a noise model and performing different noise reducing processings for each block unit. By employing such a technology, it is possible to perform a space-variant noise reducing processing, and it is possible to obtain the high quality image signals in which degradation of the edge component is little.